Radar systems are typically provided with a mechanical stabilization system for controlling the angular position of a radar antenna to thereby maintain a constant reference frame for the antenna with respect to a reference axis or plane, e.g., ground. Such mechanical stabilization is usually accomplished using servomechanisms, e.g., gimbal and drive systems. There are three types of vehicle motion that can affect the angular position of the radar antenna: roll, pitch and yaw. Roll is the side-to-side angular motion about a longitudinal, i.e., fore and aft, axis of the vehicle. Pitch is the alternating motion about an axis perpendicular to the longitudinal axis of the vehicle. Yaw is the motion of the vehicle about the vehicle's vertical axis. Position or rate gyros are typically used to sense vehicle pitch and roll, and issue position signals directly to the stabilization system to maintain the desired antenna reference.
A problem with using gimbal and drive systems for mechanical antenna stabilization is that they add significant weight to the radar system, which is of particular concern in radars intended for use on aircraft. Additionally, because of complexities associated with stabilization systems, they are prone to frequent repair and maintenance. Another problem is that servomechanisms required for roll and pitch stabilization increase the overall size of an antenna assembly with the result that within a given installation space, the area for the radiating portion of the antenna, and consequently the antenna gain and resolution, must be reduced.
A radar antenna that is rotational only about a vertical or yaw axis and not provided with roll and pitch stabilization, i.e., having a fixed platform, is referred to as being "strapped down". A strapped down antenna eliminates weight penalties and complexities associated with mechanical stabilization systems, and also reduces the space requirement for a given antenna. However, any roll and pitch experienced by a vehicle carrying the antenna is translated directly to the antenna. Movement of the antenna in roll and pitch may look to the radar system like movement of the target, and therefore, fixed platform antennas are incapable of providing accurate target position information during roll and/or pitch of the antenna, thereby preventing the radar tracking system from maintaining target tracking and correlation accuracy during antenna roll and/or pitch.
For example, if a strapped down radar is mounted on an aircraft which is located at an altitude of 1,000 feet, and is tracking an object 10 nautical miles away, a target bearing 45.degree. relative to the aircraft will have a radar indicated azimuth of 45.degree. relative during level flight. However, if the aircraft is in a roll angle of 10.degree., and therefore the radar antenna is at a 10.degree. roll angle with respect to the reference axis, the indicated azimuth will be 44.4.degree. relative. Similarly, if the aircraft is at a 10.degree. pitch angle, the indicated azimuth will be 45.6.degree. relative. The difference between the radar indicated azimuth and the actual azimuth increases with increased roll angle and pitch angle so that, for example, at a roll angle of 25.degree., the indicated azimuth will be 41.9.degree. for a target with an actual azimuth of 45.degree. relative.
The azimuth error suffered by radars which are not roll and pitch stabilized manifests itself as a "blurred" or "smeared" representation of the target on a radar display. Additionally, when roll or pitch movement of the radar antenna is rapid or of a large magnitude, the target may "jump" on the display. The azimuth error will also result in errors in the indicated target track position and velocity as provided by a radar tracking system. The tracking system position and velocity errors degrade the system's ability to maintain target track and correlation.
Typically, in a radar system used for target tracking, target position information is displayed as true azimuth rather than relative to the heading of the vehicle. In vehicles that experience rapid heading changes, e.g., aircraft, this may result in additional azimuth error. For example, if an aircraft is in a rapid or high speed turn, any delay or inaccuracy in the aircraft heading information provided to the tracking system will generate errors in target track information during the conversion from relative azimuth to true azimuth. These error in aircraft heading information typically result from delayed response of platform sensors to changes in aircraft heading, e.g., delayed response of heading rate gyros to rapid changes in aircraft heading. This azimuth error further degrades the tracking system's ability to maintain target acquisition and correlation.